In biomedical applications, tethering wires impose significant restrictions on the subject under investigation and limits the free movement. Therefore, a wireless transmitter is usually preferred to send out electro-biophysiological signals. For example, in the neural signal recording of a free-moving live subject, the recorded signal could substantially differ from that of a movement-restricted subject. In order to have concurrent access to multi-channel information in neural signal recording applications, the required transmission data rate is approximately 100 Mbps with 100 channels, even when on-the-fly signal processing and time-domain multiplexing techniques are applied.
Power consumption is also a key constraint to wireless transmitters in biomedical applications, especially when the transmitter is implanted. When powered by a battery or by wireless telemetry, the transmitter is designed to consume low power to avoid frequent battery replacement or excessive exposure of live subjects to electromagnetic waves. However, high data rate communications is difficult to realize in a limited power budget environment. For example phase-locked loop based oscillators typically are limited to generation of low data rate signals and require high power consumption. In conventional transmitters for low power biomedical applications, power consumption may be reduced by employing open-loop frequency synthesis techniques, such as open-loop voltage controlled oscillators. Such techniques, though, result in the generated frequency being inaccurate and unstable over process, voltage, and temperature variations, making demodulation at the receiver side more difficult.
Low power yet accurate frequency synthesis can be obtained through known injection locking techniques. For example, a free-running oscillator will lock to the fundamental or harmonics of an injected reference signal under the condition that the targeted harmonic is within the locking range of the oscillator. The major benefits of injection lock LC oscillators include low phase noise and low power consumption. However, the use of injection lock LC oscillators is limited to low data rate communications, even though LC oscillators are preferred for better phase noise performance. One major problem with injection lock oscillators is the variable locking time, which could be as long as three microseconds.
In biomedical applications, frequency-shift keying (FSK) modulation schemes are preferred due to their inherent superior performance in bit-error rate (BER) and interference rejection. The problem of indefinite locking time, however, still exists. For FSK modulation, conventionally the reference signal is generated by hopping from one frequency to another.
Thus, what is needed is a method and apparatus for low-power transmission of signals at a high data rate. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.